1. Field of the Invention
The presently disclosed subject matter relates to an OCT apparatus and an interference signal level control method for the OCT apparatus and, more particularly, to an OCT apparatus which detects an interference signal from an object to be measured at an appropriate level and generates optical structure information and an interference signal level control method for the OCT apparatus.
2. Description of the Related Art
Optical tomographic image acquisition apparatuses which utilize OCT (Optical Coherence Tomography) measurement are conventionally used to acquire an optical tomographic image of a living tissue. Such an optical tomographic image acquisition apparatus splits low coherent light emitted from a light source into a measuring beam and a reference beam and then multiplexes a reflected beam or a backscattered beam from an object to be measured obtained when the measuring beam is applied to the object to be measured with the reference beam. The optical tomographic image acquisition apparatus acquires an optical tomographic image on the basis of the intensity of an interference beam generated from the reflected beam and the reference beam (Japanese Patent Application Laid-Open No. 2008-128708). Hereinafter, a reflected beam and a backscattered beam from an object to be measured will be collectively referred to as reflected beams.
OCT measurement described above is roughly divided into two types: TD-OCT (Time Domain OCT) measurement and FD-OCT (Fourier Domain OCT) measurement. TD-OCT measurement is a method for acquiring a reflected beam intensity distribution corresponding to a position in a depth direction (hereinafter referred to as a depth position) of an object to be measured by measuring the intensity of an interference beam while changing the beam path length of a reference beam.
In contrast, FD-OCT measurement is a method for acquiring a reflected beam intensity distribution corresponding to a depth position by measuring the intensity of an interference beam for each spectral component of the beam without changing the beam path lengths of a reference beam and a signal beam and performing frequency analysis, typically a Fourier transform, on obtained spectral interference intensity signals using a computer. Since FD-OCT measurement does not require mechanical scanning used in TD-OCT measurement, it has recently been drawing attention as a method which allows high-speed measurement.
Typical arrangements for FD-OCT measurement include two types: an SD-OCT (Spectral Domain OCT) apparatus and an SS-OCT (Swept Source OCT) apparatus. An SD-OCT apparatus uses, as a light source, low coherent light such as an SLD (Super Luminescence Diode), an ASE (Amplified Spontaneous Emission) light source, a light source using broadband, and white light. The SD-OCT apparatus splits broadband, the low-coherent light into a measuring beam and a reference beam using a Michelson type interferometer or the like, applies the measuring beam to an object to be measured, causes a reflected beam from the object to be measured and the reference beam to interfere with each other, and decompose a resulting interference beam into frequency components using a spectrometer. The SD-OCT apparatus measures interference beam intensity for each frequency component using a detector array in which elements such as photodiodes are arranged in an array, acquires optical tomographic structure information of the object to be measured by performing a Fourier transform on each obtained spectral interference intensity signal using a computer, and forms an optical tomographic image from the optical tomographic structure information.
In contrast, an SS-OCT apparatus uses, as a light source, a laser whose optical frequency is swept in time. The SS-OCT apparatus causes a reflected beam and a reference beam to interfere with each other at each wavelength, measures time waveforms of signals corresponding to time variations in optical frequency, acquires optical tomographic structure information of an object to be measured by performing a Fourier transform on each obtained spectral interference intensity signal using a computer, and forms an optical tomographic image from the optical tomographic structure information.
OCT measurement is a method for acquiring an optical tomographic image of a specific region, as described above. In the case of endoscopes, for example, a cancerous lesion is detected by observation using a normal illumination light endoscope or a special light endoscope, and OCT measurement is performed on the region. This enables to determine to what extent the cancerous lesion has infiltrated. Additionally, two-dimensional scanning of the optical axis of a measuring beam allows to acquire three-dimensional information using depth information obtained through OCT measurement.
Since a fusion of OCT measurement and 3D computer graphics technology makes it possible to display a three-dimensional structure model with a resolution on the order of micrometers, a three-dimensional structure model obtained through OCT measurement will be referred to as an optical three-dimensional structure image (or optical three-dimensional structure information) hereinafter.
Automatization of OCT measurement is performed in various manners in conventional OCT measurement apparatuses. For example, a technique for automatically adjusting a beam path length in OCT measurement to cope with even a case where an object to be measured is separate by ultrasonically measuring the object to be measured is disclosed (Japanese Patent Application Laid-Open No. 2006-192059).